34 THE HUMAN SIDE OF TREES 



very definitely developed skeleton. One would be 

 inclined to say at first thought that a tree was all 

 skeleton. "The great mass of a tree trunk is com- 

 posed of hard, mineral-filled cells in which life is 

 extinct. They form the great bone-like structure 

 of heartwood which holds the tree aloft and en- 

 dures for long periods of time. 



Surrounding the heartwood from the tiniest root 

 to the highest branch is the one vital and vulner- 

 able organ of the tree: the cambium layer. This 

 interesting tree organ is a slimy and colourless 

 group of wood cells which covers the entire plant 

 like an undergarment. It contains within its pro- 

 toplasm the vital life principle. Here is where 

 all growth takes place. It is muscular and nervous 

 systems combined. Girdle a tree so as to expose 

 the cambium layer and everything above the wound 

 will die. 



The delicate cells of the cambium layer propagate 

 by division. On one side they produce sapwood, 

 which for a few years is used for the circulation of 

 liquids and the storage of starch, but finally through 

 continual accretions of mineral deposits solidifies 

 into heartwood. On the other side, they add bark 

 which is the tree's outer skin. 



The sap is of course the tree's blood. As a me- 

 dium of circulation it carries absorbed water and 



